The present invention relates to a method for electrically stimulating a muscle in which a stimulating signal is applied to the muscle and an electrical muscle stimulator for applying such a stimulating signal to the muscle.
It is well known that muscle contraction is caused by neural stimulation. Contraction occurs when an action potential is conducted down a nerve to a neuromuscular junction, the signal is then communicated to muscle cells and leads to the stimulation of the release of calcium ions into the cytoplasm of muscle cells which thereby modifies interactions between contractile proteins resulting in muscular contraction.
It has been long established that the application of an electrical field to muscles results in an artificially induced contraction of said muscles. Furthermore, as well as directly causing muscular contraction, electrical stimulation at specific frequencies can also modify the phenotype of a muscle. For instance, prolonged stimulation of a fast-twitch muscle with a uniform frequency of 10 Hz results in the fast-twitch muscle developing slow-twitch characteristics, namely increased endurance, but with less power than would be normal for fast-twitch muscle. Conversely, prolonged stimulation of a slow-twitch muscle with an intermittent frequency of 30-50 Hz results in the slow-twitch muscle developing fast-twitch characteristics, namely increased power, but with less endurance than would be normal for slow-twitch muscle.
It has been suggested that electrical stimulation of muscles may be a useful means of improving strength and/or endurance of incapacitated muscle (due to injury, under-use or some pathological condition). For a number of years muscles have been stimulated by Faradic stimulation delivering uniform frequencies (of around 30-50 Hz) with the aim of beneficially affecting the muscle. However, these treatments have at best been ineffective and at the worst harmful to the muscle in the long term.
UK Patent GB 2 156 682 examined the electrical discharge of nerves innervating muscle with an aim of developing a means of beneficially stimulating muscle. It discloses a method of recording electrical discharges from nerves innervating muscles. A signal generated on the basis of the recording is then used to “electrotrophically” stimulate muscle. Electrotrophic stimulation is defined as “the electrical stimulation of muscle fibre using a stimulating signal containing information effective to cause structural and/or functional change of muscle fibre without requiring the muscle fibre to respond mechanically to the stimulation”. However the stimulating signal of GB 2 156 682 is complex and difficult to generate.
Current neuromuscular products typically provide a stimulating pulsed waveform with wide variability. The shape of individual pulses may be for example symmetric biphasic pulses (for example a positive going square wave immediately followed by a negative going square wave of equal amplitude and width) or asymmetric biphasic (for example a positive going square wave immediately followed by a negative going exponentially decaying waveform. Typically clinicians are given control over a large number of parameters of the stimulation waveform, for example the pulse width (typical values are 100 ms to 300 ms), the ramp time during which the amplitude of the pulses is increased (typically 1 to 8 seconds), the frequency of the pulses (typically 2 to 150 Hz), the overall duration of a train of pulses used to contract a muscle (typically 1 to 30 seconds) and the duration of relaxation periods between successive pulse trains (typically 1 to 45 seconds). This level of variability is provided to allow clinicians to make their own choices of pulse patterns applied to the patient. Clinicians want this freedom because the effectiveness of one wavetrain pattern as compared to another is unknown and therefore clinicians tend to proceed on the basis of trial and error. Thus using the term “pulse” to signify a single electrical stimulation event in which an applied electrical voltage or current changes from a steady state baseline, each pulse generally consisting of both a single positive going phase and a single negative going phase, using the term “waveform” to represent the shape of an individual pulse, and using the term “wavetrain” to describe a series of pulses, the clinician can determine the amplitude and width of individual pulses, the waveform of individual pulses, the frequency of pulses within a single wavetrain and the duration of and spacing between successive wavetrains.
U.S. Pat. No. 5,097,833 (Campos) accurately describes known devices for maintaining or enhancing muscle tone by applying individual pulses each of which has positive and negative phases the waveforms of which are such that there is net zero charge. The waveform is not critical providing there is an equal positive and negative net charge in the positive going and negative going phases. Arranging waveforms to deliver net zero charge minimises but does not entirely remove patient discomfort. Campos states that it is known that the pulse frequency can be varied, the pulse width can be varied, and that the pulse rate and width can be preset to provide particular effects. Campos also acknowledges however that even with control of the pulse frequency or width discomfort can still arise and painful titanic contractions can be caused.
The approach suggested by Campos is to apply uniform frequency wavetrains for the whole treatment period, or to use successive wavetrains each for several minutes duration at least with successive wavetrains delivering pulses at different frequencies, the frequency within each wavetrain being uniform. Campos teaches that an evenly balanced (net zero charge) waveform may cause a “contractile imbalance” reducing the desired therapeutic effect, and notes that compensating for this by purely increasing the charge in one of the phases of the biphasic pulse will result in a charge which is not net zero and therefore will cause more discomfort. Campos seeks to overcome this problem by modifying the phasing of the pulses (i.e. by reversing the pulses such that some pulses have the positive going phase followed by the negative going phase whereas other pulses have the positive going phase following the negative going phase). Delays are also introduced between the positive and negative phases of individual pulses. According to Campos this counteracts the contractile imbalance problem whilst still maintaining a net zero charge, so as to give the optimum therapeutic effect that can be achieved with a particular wavetrain/waveform and the minimum discomfort.
Thus, Campos modifies an essentially continuous train of pulses by changing the phasing and delays between phases of individual pulses. Nevertheless Campos does use a large number of pulses which is likely to cause discomfort and requires a complex waveform generator which must be used to make complex adjustments to achieve what Campos assumes would be the optimum pattern of pulse phase reversals and delays.